It is known in the art to use intramedullary pins. U.S. Pat. No. 6,461,360B1 describes an intramedullary pin for osteosynthesis. Before insertion into a femur, the intramedullary pin has, at its distal end, in the sagittal plane, a curvature which corresponds to the counter-curvature of the femur. The proximal end substantially has a continuous curve with constant radius of curvature in the frontal plane.
U.S. Pat. No. 6,010,506 discloses a hybrid pin having different radii, all of which extend in a plane.
International publication WO 02089683 discloses an intramedullary pin using helix geometry. This structure ensures that the entry point for a pin inserted in an antegrade direction can be displaced laterally from the trochanter tip. On insertion of the pin, the pin rotates through about 90°. The rotation of the pin is influenced substantially by its helix geometry. The inner wall of the medullary space and spongiosa serve here as a guide structure.
There are disadvantages to using a pure helical geometry in varying anatomy of bones. In one instance, on reaching the end position, the distal locking holes may not be in lateral medial alignment. To correct this, the pin must be either inserted further or drawn back. The pin would rotate about its longitudinal axis as a result of either being inserted further or being drawn back. This consequently results in an undesired change of height of the locking position. On reaching the end position, the screws thus cannot be introduced centrally through the neck of the femur for locking in the head of the femur. In another instance, if only rotation is to be corrected, this can lead to a displacement of the implant depth of the pin and hence to an undesired change in the height of the locking position. If the proximal pin end is not yet completely in the bone, the pin must be inserted more deeply. This, however, results in an undesired continuation of the rotational movement. As a result of this, the optimum positions of the locking options are once again changed.